High intensity focused ultrasound systems are used in various medical ultrasound applications. During diagnostic ultrasound procedures, for example, high intensity ultrasound energy can be pulsed in a Doppler or harmonic imaging mode to propagate nonlinear waves. The harmonic frequencies of the nonlinear waves have a higher contrast to noise ratio than that of the fundamental frequency, and therefore enhance the resolution of ultrasound imaging. In therapeutic ultrasound applications, high intensity focused ultrasound energy can be radiated toward a focal region in tissue (e.g., tumors, cancerous tissue regions, bleeding spots). The accumulation of the harmonic frequencies causes rapid heating at the focal region that ablates, necrotizes, and/or otherwise damages the tissue. Rapid heating can cause boiling in tissue in the focal region. Predicting the parameters of such high intensity focused acoustic fields in situ can be important for planning treatment protocols, anticipating ultrasound-induced bioeffects in tissue, and developing safety and efficacy standards for high intensity ultrasound.
A process known as “derating” can be used to estimate the parameters of an acoustic field in situ. During a typical derating process, acoustic field measurements are taken in water at low level ultrasound source outputs. The measured values are then linearly extrapolated to account (1) for higher source outputs used in medical procedures and (2) for tissue attenuation. For example, a measured source pressure can be scaled linearly to obtain the focal pressure amplitude in water, and the linearly scaled focal pressure can then be derated by a compensation factor that depends on the propagation path (i.e., the focal distance) and the linear attenuation coefficient of tissue to determine the focal pressure in tissue.
When working with high intensity diagnostic and therapeutic ultrasound applications that produce nonlinear acoustic fields, the assumption of linear acoustic propagation introduces errors into the derating process. For example, the wave intensity at the focus is not a quadratic function of the pressure amplitude at the fundamental frequency, but instead consists of contributions from all of the harmonics. Similarly, the heating rate at the focus is not proportional to the intensity at the focus due to the contribution of more readily absorbed higher frequency components. Accordingly, linear derating is generally unsuitable for the estimation of nonlinear acoustic field parameters in tissue.